STAR-Transformer: A Spatio-temporal Cross Attention Transformer for Human Action Recognition

In action recognition, although the combination of spatio-temporal videos and skeleton features can improve the recognition performance, a separate model and balancing feature representation for cross-modal data are required. To solve these problems, we propose Spatio-TemporAl cRoss (STAR)-transformer, which can effectively represent two cross-modal features as a recognizable vector. First, from the input video and skeleton sequence, video frames are output as global grid tokens and skeletons are output as joint map tokens, respectively. These tokens are then aggregated into multi-class tokens and input into STAR-transformer. The STAR-transformer encoder layer consists of a full self-attention (FAttn) module and a proposed zigzag spatio-temporal attention (ZAttn) module. Similarly, the continuous decoder consists of a FAttn module and a proposed binary spatio-temporal attention (BAttn) module. STAR-transformer learns an efficient multi-feature representation of the spatio-temporal features by properly arranging pairings of the FAttn, ZAttn, and BAttn modules. Experimental results on the Penn-Action, NTU RGB+D 60, and 120 datasets show that the proposed method achieves a promising improvement in performance in comparison to previous state-of-the-art methods.